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BUG: KCSAN: data-race in __add_to_page_cache_locked / nr_blockdev_pages

write to 0xffff888101eb8790 of 8 bytes by task 4801 on cpu 0:
 __add_to_page_cache_locked+0x3ad/0x6e0 mm/filemap.c:887
 add_to_page_cache_lru+0xa0/0x1b0 mm/filemap.c:940
 pagecache_get_page+0x6a0/0xa30 mm/filemap.c:1808
 find_or_create_page include/linux/pagemap.h:405 [inline]
 grow_dev_page+0x78/0x400 fs/buffer.c:974
 grow_buffers fs/buffer.c:1043 [inline]
 __getblk_slow fs/buffer.c:1070 [inline]
 __getblk_gfp+0x13b/0x1f0 fs/buffer.c:1354
 __getblk include/linux/buffer_head.h:380 [inline]
 jbd2_journal_get_descriptor_buffer+0x9d/0x1c0 fs/jbd2/journal.c:1022
 jbd2_journal_commit_transaction+0xac5/0x3290 fs/jbd2/commit.c:656
 kjournald2+0x263/0x480 fs/jbd2/journal.c:213
 kthread+0x20b/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffff888101eb8790 of 8 bytes by task 3586 on cpu 1:
 nr_blockdev_pages+0x7c/0xd0 fs/block_dev.c:942
 si_meminfo+0x3a/0x60 mm/page_alloc.c:5407
 update_balloon_stats drivers/virtio/virtio_balloon.c:330 [inline]
 stats_handle_request drivers/virtio/virtio_balloon.c:386 [inline]
 update_balloon_stats_func+0x41/0x2e0 drivers/virtio/virtio_balloon.c:472
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2275
 worker_thread+0x616/0xa70 kernel/workqueue.c:2421
 kthread+0x20b/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 3586 Comm: kworker/1:2 Not tainted 5.12.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events_freezable update_balloon_stats_func
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